Polypropylene nonwoven fabrics have excellent properties such as breathability and softness and are used as hygiene materials including diapers and sanitary items. However, further improvements in their properties have been required. For example, polypropylene nonwoven fabrics improved in softness, bulkiness and mechanical strength are desired.
To obtain nonwoven fabrics having excellent softness and bulkiness, various methods have been proposed in which nonwoven fabrics are formed of crimped polypropylene fibers. For example, Patent Document 1 discloses nonwoven fabrics that comprise conjugated fibers having a crimpable cross-sectional configuration wherein the conjugated fibers comprise a first component comprising propylene polymer and a second component comprising polypropylene with different properties from the first component. The second polypropylene is selected from the group consisting of high MFR polypropylenes, low polydispersity polypropylenes, amorphous polypropylenes and elastic (elastomeric) polypropylenes. According to the disclosure, melt spinning the first component and the second component having different properties from each other and attenuating the resultant conjugated fibers give crimped fibers capable of forming nonwoven fabrics with excellent softness and elastic properties.
Patent Document 2 discloses nonwoven fabrics that comprise parallel type crimped conjugated fibers comprising ethylene/propylene random copolymer and polypropylene.
In Patent Document 1, crimped conjugated fibers are obtained from a combination of polypropylenes having dissimilar properties. In detail, Example 1 discloses a combination of polypropylenes having differing MFR and molecular weight distribution in which parallel type conjugated fibers are formed from a first polypropylene having an MFR of 35 and a polydispersity number of 3 and a second polypropylene having an MFR of 25 and a polydispersity number of 2.
The present inventors preliminarily produced conjugated fibers according to the disclosure of the patent document. It has been then found that spinnability and crimp properties are insufficient.
With regard to the parallel type crimped conjugated fibers of Patent Document 2, crimps are produced utilizing differing crystallization degrees of the ethylene/propylene random copolymer and the polypropylene that are spun. However, nonwoven fabrics obtained from two polymers with differing crystallization degrees have a performance that is an average of the two polymers.
In the background art as described above, the production of crimped polypropylene fibers entails a combination of two propylene polymers having greatly different physical properties, specifically MFR. In the case of identical MFR, a combination should consist of a propylene homopolymer and a propylene/α-olefin random copolymer having different melting points (crystallization temperatures). It has been considered difficult to obtain crimped conjugated fibers from a combination of similar polymers, in particular propylene homopolymers or propylene/α-olefin random copolymers having similar MFR (i.e., melt flow properties).
Further, the use of two propylene polymers greatly differing in melting point restricts production conditions such as melting temperature. In the case of propylene polymers having greatly different MFR, the molten fibers of the propylene polymers can be curved immediately after spun from the nozzle and can adhere and contaminate the nozzle face.